In optical disk drive storage devices, information is written to and read off of a rotating disk via a focused laser beam from a disk reading mechanism. Information is stored on the disk in binary form on concentric circles called tracks. Because the rotating disk is subject to axial and radial accelerations, a driving mechanism is necessary to dynamically position an optical element which focuses the laser beam on the desired track. The positioning of the optical element is normally achieved by utilizing two feedback control loops, one for controlling motion in the track axis, and one for controlling motion in the focus axis. The driving mechanism (actuator) serves to support the optical element and convert the feedback control signal into mechanical motion in both the track and focus axes. The control systems often lack authority to position the actuator when subjected to shock and vibration, in which case the loop is broken and control of the actuator is lost. When control of the actuator is lost, it would be advantageous to have a secondary set of control loops which can be utilized to regain control of the driving element more quickly than if only the primary track and focus control loops were used.
Many optical element positioning mechanisms exist. With most two axis actuators, a pair of track axis driving coils for creating movement in one axis, and a focus axis driving coil for creating movement in a second axis, are utilized. These coils are placed on the support structure of the optical element. The optical element is then translated in either axis by energizing the appropriate driving coil which creates a force to move the optical element. A problem exists with this method of positioning the optical element. Since both driving coils are located on the optical element structure, more mass must be overcome in order to move the optical element in the track axis. This translates into more force and power required during operation. It is desirable to have efficient operation and minimize power used by the actuator.
Some actuators on the market utilize a single axis secondary position detection mechanism. The most common detection approach is to add an appendage to the movable portion of the actuator and shine a light against it, creating a shadow on the two element photodetector. If the photodetector elements are unevenly illuminated, a differential signal proportional to the displacement of the optical element is produced. The problem with this existing mechanism is that position detection is possible in only a single axis.